The invention relates to a dual band antenna for a handset. Such an antenna includes a metallic plate or layer acting as ground plane for the antenna, a resonator plate or layer acting as radiating element(s), and a feeding point supplying the signal to the antenna.
The applicant launched recently a new GSM dual band phone named Nokia 3210(trademark). This phone has a dielectric antenna body covered by a metallic pattern forming two radiating elementsxe2x80x94one for each band. The dielectric antenna body is inside the phone snapped onto a metallic shield acting as resonator plane. The antenna used in Nokia 3210(trademark) is a PIFA (Planar Inverted F-Antennas) antenna and is described in GB 9828533.1, GB 9828364.1, and GB 9828535.6xe2x80x94all filed in December 1998.
WO 95/24746 describes a single band internal antenna having a dielectric body coated with a metallic layer on two substantially parallel surfaces.
U.S. Pat. No. 5,764,190 describes a capacity loaded PIFA according to which an extra plate is interposed in between the ground plane and the radiating element.
U.S. Pat. No. 5,764,190 describes how to provide a longitudinal slit in the resonator layer in order to obtain two radiating elements. A capacitive feeding concept is used.
A letter by Z. D. Lui and P. S. Hall, xe2x80x9cDual-Frequency Planar Inverted-F Antennaxe2x80x9d, is published in IEEE Transactions on Antennas and Propagation, October 1997, Volume 45, Number 10. This letter describes a number of solutionsxe2x80x94one of these having a rectangular patch for the 900 MHz band. This patch is provided with an L-shaped slot separating one quarter of the 900 MHz band for acting as resonating element in 1800 MHz band. GSM works in the 900 MHz band (uplink: 890-915 MHz (mobile to base-station), and downlink: 935-960 MHz (base-station to mobile)) and in the 1800 MHz band (uplink: 1710-1785 MHz (mobile to base-station), and downlink: 1805-1880 MHz (base-station to mobile)).
An object of the invention is to provide a dual band antenna having a reduced overall size.
This object is achieved by a dual antenna device having a first conducting layer acting as resonator plane for the antenna device, a second conducting layer, that is substantially parallel with the first conducting layer, and acting as ground plane, and a dielectric body on which said first conducting layer is provided. The first conducting layer comprises two branches, and both branches will contribute to the matching of the antenna device in both hands. Hereby the full patch area may be used either for radiating an electromagnetic field or for mating the antenna.
Preferably the one of said two branches is quarter-wave resonant in a first one of said two bands, and half-way resonant in a second one of said two bands, while the second one of said two branches provides a resonant matching in said first one of said two bands, and will appear as a quarter-wave resonant stub in said second one of said two bands. When the antenna device is used in a GSM dual band phone the two bands will have center frequencies in approximately 920 MHz and in approximately 1800 MHz, respectively.
By placing the strips of the feeding means in parallel close together the Q-value of the antenna will be reduced and hence the bandwidth of the antenna will be increased. Also this arrangement provides better flexibility for the patch layout since the feed occupies less area on the patch.
According to the referred embodiment the antenna elements constituted by the branches have been folded in order to reduce the RF coupling between the two branches. This can be done by locating the open ends away from each other, as well as aligning the currents of the two at 90 degrees angle. Hereby the capacitive coupling between the open ends of the stubs (electrical field) will be reduced. Furthermore the inductive coupling between the branches where the currents are strong (close to the feed and at 1800 MHz at the middle of the 900 MHz as well) will be reduced. Locating the feed close to the edge of the PCB will also increase bandwidth.
Besides minimizing the coupling voltage/voltage and current/current of the two branches, the layout distributes the currents in a large area of the patch, which is desirable.